A cubic boron nitride sintered body has high hardness and excellent thermal conductivity, so that it has been used as a cutting tool for hardened steel or heat-resistant alloy, etc. In recent years, for the purpose of improving machining efficiency, a coated cubic boron nitride sintered body tool in which a coating layer is coated on the surface of a cubic boron nitride sintered body substrate has been used.
Prior art techniques of the coated cubic boron nitride sintered body tool include a composite highly hard material for hardened steel-cutting tool in which the surface of a substrate comprising a CBN sintered body containing 20% by volume or more of cubic type boron nitride is coated with a membrane of TiAlN wherein an atomic ratio X of Ti/(Ti+Al) is 0.3≦X≦0.5 (for example, see Patent Literature 1.). However, when the material is used as a cutting tool and a cutting speed becomes high speed, that the temperature of a blade edge of the tool becomes high, whereby there are produced problems that the TiAlN membrane is thermally decomposed and oxidized, and the coated membrane is peeled off from the surface of the substrate to cause abnormal wear.
Further, there is a cutting tool made of a surface-coated cubic boron nitride-based ultra-high pressure sintered material wherein a hard coating layer comprising a lower layer of a complex nitride layer of Ti and Al satisfying [Ti1-XAlX]N (X represents 0.4 to 0.6), and an upper layer which has an alternately laminated layer structure comprising a thin layer A of a complex nitride layer of Ti and Al satisfying
[Ti1-XAlX]N (X represents 0.4 to 0.6) and a thin layer B of a Cr nitride layer is formed by deposition (for example, see Patent Literature 2.). However, this cutting tool involves a problem that it causes abnormal wear due to breakage of the hard coating layer since Cr in the upper layer is diffused into the hard coating layer accompanied by increase of temperature of the blade edge of the tool in machining of a hardened steel.    [Patent Literature 1] JP 3866305B    [Patent Literature 2] JP 2008-18504A